Handbook of Electrical Engineering

EARTHING AND SCREENING 375

Figure 13.13 Mutual coupling between spaced out cables.

In these examples the situation of interest is a combination of a single conductor above a plane
and another single conductor above the same plane but running in a cylinder or screen, as shown in
Figure 13.13.

The source of interference in the example is a three-phase cable in which unbalanced currents
flow. The currents that are unbalanced can be replaced by one equivalent current, which is the sum
or resultant of all the three phase currents. The three-phase cable is assumed to be armoured, which
is generally the case, and the armouring is assumed to be earthed at one or both ends. Earthing the
armour reduces the external electric field to zero, and so only mutual inductive coupling needs to
be considered.

The equivalent circuit of the various conductors and screening is shown in Figure 13.14.

Where:

For the interference source cable

I 3 is the three-phase resultant interference current source.

R 3 is the resistance of the source circuit.

L 3 is the self-inductance of the source circuit.

For the signal instrumentation cable

Rs is the resistance of the cable screen.

Ls is the self-inductance of the cable screen.

Rs is the resistance of the cable core.